1. Field of the Invention
This invention relates to storage-stable, concentrated aqueous solutions of disazo acid dyes.
2. Description of the Prior Art
The use of direct, acid and basic dyes as concentrated aqueous solutions, often referred to in the art as liquid dyes, has become commercially important relatively recently for dyeing paper and textile goods. The chief advantages of such liquid dyes over dye powders are the avoidance of dusting problems in handling and the great ease and rapidity of metering of the liquid dyes. Ideally, a liquid dye consists of an aqueous solution of the dye, thus avoiding disposal or recycling problems associated with solutions containing organic solvents. Dyes containing at least two ionizable groups, for example, sulfonic acid or quaternary ammonium groups, often have sufficient water solubility to give aqueous solutions of the desired concentration for use as liquid dyes, that is, at least 10 weight %. In some cases, the requisite solubility of polysulfonated dyes can be obtained by judicious choice of the cation associated therewith. For example, U.S. Pat. No. 3,600,376 discloses aqueous liquid dyes containing 10-20 weight % of the lithium salts of various tetrasulfonated disazo dyes. U.S. Pat. No. 3,681,320 discloses aqueous solutions of the triethanolamine salts of such dyes. It is much more common in the art, however, to prepare liquid dyes which consist in part, or even predominantly, of a solubilizing aid which is usually a water miscible organic liquid. British Pat. No. 1,202,798 discloses aqueous preparations of the sodium salts of polysulfonated azo dyes to which an acid amide, preferably urea, has been added. U.S. Pat. No. 3,551,088 discloses aqueous solutions of tri- and tetrasulfonated disazo dyes containing 20-30 weight % of dye and 10-25 weight % of dimethyl sulfoxide, the latter being used as a solubilizing aid. U.S. Pat. No. 3,346,322 discloses concentrated organic or aqueous organic solutions of water soluble aliphatic carboxylic acid salts of basic dyes, the organic liquid being selected from glycols and ethers thereof, amides, butyrolactone, acetonitrile, tetrahydrofuran, dioxane, dimethylsulfoxide and N-methylpyrrolidone.
In contrast to the above, the sodium salts of monosulfonated disazo dyes generally have limited solubility in cold water; they are seldom more than 1 weight % soluble and often their solubilities may be as low as a few hundreths of 1%. Although the solubilities of some monosulfonated dyes can be markedly improved by employing lithium, ammonium or triethanolammonium as the cation associated therewith, the solubilities realized are not sufficient to produce a concentrated, stable aqueous solution of commercial acceptability.